It's not "definitionally false", you just have sexist views that have become so internalized and integral to your world view that you can't even see it.

I would say “factually false and fairly trivially obviously so”, rather than “definitionally false”, since it logically could be true even though it isn’t, but otherwise...

You'll have to walk me through how it's sexist.

As for being definitionally false, it is. A claim of precisely two genders, or even of precisely two sexes, is medically incorrect.